1. Field of the Invention
This invention relates to a reverse-charge prevention apparatus including a 1st breaker for connecting or disconnecting an electric power system to or from a distributed electric power apparatus, in which reverse-charge from the distributed electric power apparatus to the electric power system is prevented by switching off the 1st breaker when the distributed electric power apparatus is disconnected from the electric power system by switching off a 2nd breaker disposed in the electric power system.
2. Description of the Prior Art
In recent years, in order to harness natural energy or improve the efficiency of energy use, there has been developed an inverter type of distributed electric power apparatus in which direct-current electric power output by a solar battery or a fuel battery is transformed into alternating-current electric power by means of an inverter or a so-called "cogeneration" type of distributed electric power apparatus in which alternating-current electric power is generated by linking an engine to a generator. By connecting such a distributed electric power apparatus to the electric power system (constituted of power stations, transmission lines, substations, distribution lines, etc.), an electricity shortage can be filled up by the electric power of the electric power system to constantly supply electric power when an accident occurs to the distributed electric power apparatus or electric power generated by only the distributed electric power apparatus cannot sufficiently meet electric power demand.
However, the following problem might occur in a case in which the distributed power apparatus and the electric power system are connected to each other.
That is, there is a case in which, before construction work or repairs are carried out for the electric power system, breakers are switched off to stop supplying electric power. If the distributed electric power apparatus is connected to a distribution line, a voltage of the distributed electric power apparatus is applied to the distribution line so that a state of reverse-charge is made even if the electric power system is intercepted (in other words, even if a breaker of the distribution line is switched off to stop supplying electricity). For this reason, a worker carrying out the construction work or the like on the distribution line has a lot of danger of receiving an electric shock due to the reverse-charge.
Accordingly, a method is conventionally adopted in which the distributed electric power apparatus is provided with an over-under voltage relay (about .+-.10% with respect to a reference voltage), an abnormal frequency relay (about .+-.1 Hz) with respect to a reference frequency), or the like and is disconnected from the electric power system on the assumption that the circuit of the electric power system is opened (broken) if an allowable value is exceeded by a difference between the reference voltage and a voltage generated when the circuit of the electric power system is opened or a difference between the reference frequency and a frequency generated when the same is opened.
However, in the above method, if the electric power from the distributed electric power apparatus and a load needed by consumers are balanced with each other, there occurs a problem that the openness of the circuit of the electric power system cannot be certainly detected since variations in voltage and frequency are too small to be detected by the above relays.
To solve the problem, in view of the fact that the inverter type of distributed electric power apparatus has a variation different from a periodic variation peculiar to the electric power system in a period of an outputted alternating-current voltage, a reverse-charge prevention apparatus is proposed in which, by judging that a supply of power is cut off from the electric power system when a variation in the period of the alternating-current voltage is observed on the side of the distributed electric power apparatus, the distributed electric power apparatus is disconnected from the electric power system so that the reverse-charge from the distributed electric power apparatus to the electric power system is prevented.
However, there is a case in which the inverter type of distributed electric power apparatus has no variation different from the periodic variation peculiar to the electric power system if use is made of not only a still type of generator, such as a solar generator, but also a rotary generator provided with a diesel engine for cogeneration in the distributed electric power apparatus.
In this case, as a further problem, it is difficult to judge whether the electric power system is opened or not, hence that the reverse-charge cannot be prevented.
To solve the problem, another reverse-charge prevention apparatus is proposed in which a slight periodic variation different from a periodic variation peculiar to the electric power system is first given to the alternating electric power outputted by the distributed electric power apparatus so that a variable period same in period to the slight periodic variation given thereto appears in a period of the alternating voltage outputted by the distributed electric power apparatus when the electric power system is opened and then the distributed electric power apparatus is disconnected from the electric power system when the variable period of the alternating voltage is extracted.
However, since a variable period peculiar to the electric power system depends on each area where electric power is used and further variable periods of the cogeneration types of distributed electric power apparatuses are different from each other, it is impossible to equally determine respective variable periods given to the alternating-current electric power of the distributed electric power apparatuses and, in a certain area, it is difficult to judge whether the electric power system is opened or not.
This invention was made in view of the above problems and aims to provide a reverse-charge prevention apparatus in which the openness of the electric power system is certainly detected independently of a variable period peculiar to the electric power system or a variable period of the distributed electric power apparatus, so that the reverse-charge from the distributed electric power apparatus to the electric power system is prevented.